Need to Upgrade RAM for XPS L501x

Hello reader,

I purchased my Dell L501x on 2nd August 2011. It came with 3GB Ram. Now I find an urgent need to upgrade my RAM to 8GB (maximum possible RAM supported by the motherboard) so I could work on my thesis smoothly which requires heavy simulations on MATLAB. I have tried several RAM with the laptop and I have found none compatible with the motherboard. My university IT support was also confused. Please suggest me a perfectly compatible RAM so I could upgrade my system. Below are some of the details of my purchase.

I would be really, truly grateful if someone helps me find a compatible RAM from or outside Dell store :)

Internet Receipt No: <ADMIN NOTE: Non-public info removed as per TOU>

Booking Reference: <ADMIN NOTE: Non-public info removed as per TOU>

Order Number: <ADMIN NOTE: Order number removed per privacy policy>

Service Tag:<ADMIN NOTE: Service tag removed per privacy policy>

Email address used for purchase: <ADMIN NOTE: Email id removed per privacy policy>

Absar